The Civil Aviation Authority of Singapore (CAAS) said on Wednesday that these two countries both have comprehensive public health surveillance systems and have successfully controlled the spread of the COVID-19 outbreak.

"The risk of importation from these countries is low," said the authority, noting that over the last 28 days, Vietnam had zero local COVID-19 cases while Australia (excluding Victoria state) had a low virus incidence rate of 0.02 cases per 100,000 people.

The two countries join New Zealand and Brunei as the four countries that Singapore has unilaterally opened its borders to, as the country bids to revive its tourism and travel markets.

Visitors from Australia and Vietnam will be able to apply for the Air Travel Pass for entry into Singapore from Thursday. The earliest day they will be able to come into Singapore will be Oct. 8.

The Air Travel Pass scheme allows for all forms of short-term travel, including leisure travel. This is in contrast to reciprocal green lane arrangements between countries, which are usually for essential business and official travel.

Visitors will have to take a swab test upon arrival, and can go about their activities after they get a negative test result. They will have to use the TraceTogether app for the duration of their stay here, among other conditions.

CAAS also said that Singapore citizens, permanent residents and long-term pass holders returning from Australia (excluding Victoria state) and Vietnam will be able take a COVID-19 test upon arrival.

This will replace the current requirement of a seven-day stay-home notice with a test before the end of the notice.

CAAS added that it has updated Singapore’s travel advisory to allow for travel to Australia (excluding Victoria) and Vietnam.

It advised travelers to check the entry requirements imposed by these countries and to take necessary precautions if travelling.

Australia currently bans all overseas travel, and those who want to do so have to apply for an exemption. In addition, foreigners are not allowed to enter the country at the moment. 

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Meanwhile, Vietnam has temporarily suspended entry of all foreigners and overseas Vietnamese via commercial flights. It will only approve entry for some foreigners, such as those with diplomatic passports, on a case-by-case basis.

Since border restrictions to visitors from Brunei and New Zealand were lifted on Sept. 8, CAAS has approved 331 applications from travelers from the two countries.

As of 5 p.m. on Wednesday, 136 visitors have arrived so far. None of them tested positive for COVID-19 upon arrival.

Transport Minister Ong Ye Kung said last month that the unilateral reopening of borders to Brunei and New Zealand was a "small, cautious" step that can resuscitate Changi Airport.

In a Facebook post on Wednesday, Ong  said he has spoken to Michael McCormack, Australia’s deputy prime minister and Tao Thi Than Huong, Vietnam’s ambassador to Singapore, about the move.

"They were appreciative of Singapore’s decision, and when conditions are right, I believe they will consider reciprocating the lifting of restrictions to travelers from Singapore," he said.

The minister added that Singapore’s move to further open up was another step in resuscitating Changi Airport.

The Changi air hub is a crucial part of Singapore's economy, contributing more than 5 percent of Singapore's gross domestic product and employing 192,000 people before the pandemic.

Now, it is among the worst-hit segments of the economy, with the transportation and storage sector shrinking at an annualized rate of 80 per cent in the second quarter of the year, compared with the previous quarter.